This invention relates generally to semiconductor memory devices, and particularly to write line architectures for magnetoresistive random access memory (MRAM) storage cells.
Semiconductor devices are used for integrated circuits in a wide variety of electrical and electronic applications, such as computers, cellular telephones, radios, and televisions. One particular type semiconductor device is a semiconductor storage device, such as random access memory (RAM) and flash memory. These semiconductor storage devices use an electrical charge to store information.
A recent development in semiconductor memory devices involves spin electronics, which combines traditional semiconductor technology and magnetism. Rather than using an electrical charge to indicate the presence of a binary xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, the spin of an electron is used. An example of such a spin electronic device is a magnetoresistive random access memory (MRAM) storage device, which includes conductive lines positioned perpendicular to one another in different metal layers. The place where the conductive lines intersect is known as a cross-point. In between the perpendicular conductive lines is a magnetic stack. The magnetic stack is placed at the cross-point, sandwiched between the conductive lines.
An electrical current flowing through one of the conductive lines induces a magnetic field around the conductive line. The induced magnetic field can align (or orient) the alignment (or orientation) of magnetic dipoles in the magnetic stack. The right hand rule is a way to determine the direction of a magnetic field induced by a current flowing in a particular direction. The right hand rule is well understood by those of ordinary skill in the art of the present invention.
A different current flowing through the other conductive line induces another magnetic field and can realign the polarity of the magnetic field in the magnetic stack. Binary information, represented as a xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, is stored as different alignments of the magnetic dipoles in the magnetic stack. A current of sufficient strength flowing through one of the conductive lines is sufficient to destroy the contents of the magnetic stacks coupled to it. However, currents flowing through both conductive lines are required to selectively program a particular magnetic stack.
The alignment of the magnetic dipoles in the magnetic stack changes the electrical resistance of the magnetic stack. For example, if a binary xe2x80x9c0xe2x80x9d is stored in the magnetic stack, the resistance of the magnetic stack will be different from the resistance of the same magnetic stack if a binary xe2x80x9c1xe2x80x9d is stored in the magnetic stack. It is the resistance of the magnetic stack that is detected and determines the logical value stored therein.
However, due to manufacturing variations, different magnetic stacks require magnetic fields of differing strength to realign the magnetic dipoles. For example, one magnetic stack may require a stronger magnetic field to realign its magnetic dipoles than another magnetic stack. To ensure that any magnetic stack can be realigned, magnetic fields of sufficient strength much be used. However, if the magnetic fields are too great, then overly sensitive magnetic stacks may be unintentionally realigned, resulting in the unintended destruction of data.
Because MRAM devices operate differently than traditional semiconductor memory devices, they introduce design and manufacturing challenges. A need has therefore arisen for a write line architecture to minimize the probability of erroneously realigning the magnetic dipoles of unselected magnetic stacks.
In one aspect, the present invention provides a MRAM storage device featuring a segmented write line architecture comprising a plurality of MRAM memory cells arranged into segments, a first master wordline that is decodable using address bits to select a single local wordline, a second master wordline that is decodable using address bits to select the single local wordline, a return line coupled to the local wordline, wherein the local wordline is coupled to a segment of MRAM memory cells and the first and second master wordlines provide a current of desired magnitude to align the MRAM memory cells and the return line is used to sink the current used to align the MRAM memory cells.
The present invention provides a number of advantages. For example, use of a preferred embodiment of the present invention permits the realignment of a selected magnetic stack with educed probability of inadvertently realigning an unselected magnetic stack by eliminating any unselected magnetic stacks seeing (being exposed to) the hard axis field. Also, the present invention provides a nearly invariant write current along the write line due to a decrease in the amount of parasitic write current encountered by the write current. An invariant write current increases the certainty of the occurrence of a realignment operation. The invariant write current also permits a reduced write margin to help ensure that unselected cells are not unintentionally realigned.
Additionally, since the present invention eliminates any unselected magnetic stacks being exposed to the hard axis field, a higher hard axis field can be used to ensure the realignment of the magnetic stack with a lower soft axis field to further reduce the probability of realigning an unselected magnetic stack due to exposure to the soft axis field.
Also, the plurality of write current lines permits a reduction in the size of the current sources, making them easier to place in the layout of the MRAM memory array.